In most oil wells, water enters the well and is recovered together with the oil. Furthermore, as the well ages the amount or cut of recovered water generally increases. On average, about 3 tons of water per one ton of oil are pumped out of depleting oil formations [1]. The size of the water cut produced by the well has a substantial effect on the economics of well operation.
Thus methods for limiting water inflow have been developed. These make it possible to reduce substantially the cost of oil recovery and to increase the amount of oil extracted from the formation.
One method of limiting the inflow of water involves the injection into the well of liquids capable of selectively reducing the flow of water without impeding the flow of oil [2]. Ideally, during injection into the well the liquid has a relatively low viscosity. Subsequently the liquid should form a gel “plug” in those zones of the well from which there is an inflow of water, but gelation should not occur upon contact of the liquid with oil.
The active component of gelable liquids can be provided by hydrophobically associating substances which are capable of forming physical gels in aqueous media [2, 3]. Such substances may be e.g. hydrophobically modified polymers, viscoelastic surfactants and polymer/surfactant complexes [2, 3].
The formation of physical gels from hydrophobically modified polymers is described in papers [4-6]. The formation of physical gels from viscoelastic surfactants is described in papers [7-9]. Gelation in polymer/surfactant complexes is described in papers [10-12].
A problem associated with gelable liquids intended to have selective placement capability and based on hydrophobically associating substances is that these substances tend to form physical gels immediately after being introduced to water. It is then difficult to pump the liquid into the well because formation of a physical gel has already commenced, increasing the viscosity of the system. Another problem is that contact between the physical gel and hydrocarbons may not result in destruction of the gel, leading to plugging of oil-bearing formations. The latter problem is particularly associated with gels formed by hydrophobically modified polymers. Thus it would be desirable to be able to: (1) sufficiently slow down the process of gelation in aqueous media such that the gelable liquid has a low viscosity during injection, (2) nonetheless form a gel downhole on contact between the liquid and water, and (3) suppress gelation on contact between the liquid and oil.
Several methods for slowing gelation have been proposed for gelable liquids comprising viscoelastic surfactants [3]. However, as far as the present inventors are aware, methods for controlling the speed of formation of physical gels in gelable liquids comprising hydrophobically modified polymers are largely undeveloped.
As is shown in papers [13-15] from other technical fields, hydrophobic aggregates formed in diluted solutions of hydrophobically modified polymers and polymer/surfactant complexes may be destroyed in water-organic mediums. For example, it is shown in papers [13, 14] that in dilute solutions of hydrophobically modified polyacrylamide, the hydrophobic aggregates are destroyed when 20-50 vol. % of acetonitrile is added to the water. This solvent is used during the determination of the molecular weight of individual macromolecules by gel-permeation chromatography (GPC). Similar behaviour is observed in polymer/surfactant complexes. Paper [15] discloses that in water/ethanol and water/isopropanol mixtures, containing 40-50 vol. % of alcohol, destruction of the polymer/surfactant complexes takes place because of a weakening of hydrophobic interactions.